Generally, the process of electrophotographic printing includes charging a photoconductive member to a substantially uniform potential so as to sensitize the surface thereof. The charged portion of the photoconductive surface is exposed to a light image from either a scanning laser beam or an original document being reproduced. This records an electrostatic latent image on the photoconductive surface. After the electrostatic latent image is recorded on the photoconductive surface, the latent image is developed. Two component and single component developer materials are commonly used for development. A typical two component developer comprises magnetic carrier granules having toner particles adhering triboelectrically thereto. A single component developer material typically comprises toner particles. Toner particles are attracted to the latent image forming a toner powder image on the photoconductive surface, the toner powder image is subsequently transferred to a copy sheet, and finally, the toner powder image is heated to permanently fuse it to the copy sheet in image configuration.
The electrophotographic marking process given above can be modified to produce color images. One color electrophotographic marking process, called image on image processing, superimposes toner powder images of different color toners onto the photoreceptor prior to the transfer of the composite toner powder image onto the substrate. While image on image process is beneficial, it has several problems. For example, when recharging the photoreceptor in preparation for creating another color toner powder image, it is important to level the voltages between the previously toned and the untoned areas of the photoreceptor.
In the application of the toner to the latent electrostatic images contained on the charge-retentive surface, it is necessary to transport the toner from a developer housing to the surface. A limitation of conventional xerographic development systems, including both magnetic brush and single component, is the inability to deliver toner (i.e. charged pigment) to the latent images without creating large adhesive forces between the toner and the conveyor on which the toner rests and which transports the toner to latent images. As will be appreciated, large fluctuation in the adhesive forces that cause the pigment to tenaciously adhere to the carrier severely limits the sensitivity of the developer system, thereby necessitating higher contrast voltages forming the images. Accordingly, it is desirable to reduce the large adhesion, particularly in connection with latent images formed by contrasting voltages.
In order to minimize the adhesive forces, there is provided, in the preferred embodiment of the invention, a toner conveyor including means for generating traveling electrostatic waves which can constantly move the toner about the surface of the conveyor with minimal static contact therewith.
Traveling waves have been employed for transporting toner particles in a development system, for example U.S. Pat. No. 4,647,179 to Schmidlin which is hereby incorporated by reference. In that patent, the traveling wave is generated by alternating voltages of three or more phases applied to a linear array of conductors placed about the outer periphery of the conveyor. The force F for moving the toner about the conveyor is equal qE.sub.t where q is the charge on the toner and E.sub.t is the tangential field supplied by a multi-phase AC voltage applied to the array of conductors.
Traveling wave devices have been proposed for a number of years to transport, separate and deliver charged particles to a latent electrostatic image. Some of the other reasons this is an attractive approach include absence of moving mechanical parts, control of the toner position, long and stable development zones, and architectural flexibility. A semiconductive overcoat may be desirable on the grid providing a smooth surface for the toner motion and also a possible charge relaxation channel. Previous work has shown that various modes of charged particle transport are possible. The so-called synchronous modes of the electrostatic traveling wave transport have been found and indicated as appropriate to facilitate the toner transport that can be used for xerographic development systems. In those modes, the toner particles move along the carrying surface with the traveling wave phase velocity v.sub.ph =.omega./k where .omega. and k are the frequency and the wavevector of the wave respectively. This velocity is achieved through the action of the longitudinal (x) component of the electrostatic force while the normal (z) component of the force on the average contains the toners near the carrying surface.
In the other, so-called "curtain" or asynchronous mode, toners would be effectively repelled by the wave from the surface and could be retained only by an external force such as the gravity or another externally applied electric field. In the absence of the latter, the toners would be very loose and subject to emissions. Transport in this mode ordinarily occurs with velocities much lower than v.sub.ph.
While being transported in synchronous modes, the toner particles, although moving on the average along the surface, still find themselves in intimate contact with it for appreciable periods of time. At the same time, while in the development zone such toners can be effectively screened by the traveling wave from the development fields.